4,894 research outputs found
Particle collisions in the lower dimensional rotating black hole space-time with the cosmological constant
In this paper, we study the effect of ultra-high energy collisions of two
particles with different energies near the horizon of a 2+1 dimensional BTZ
black hole (BSW effect). We find that the particle with the critical angular
momentum could exist inside the outer horizon of BTZ black hole regardless of
the particle energy. Therefore, for the non-extremal BTZ black hole, the BSW
process is possible on the inner horizon with the fine tuning of parameters
which are characterized by the motion of particle. While for the extremal BTZ
black hole, the particle with the critical angular momentum could only exist on
the degenerate horizon, and the BSW process could also happen there.Comment: 12 pages,3 figure
The prediction of using LHAASO's cosmic-ray electron measurements to constrain decaying heavy dark matter
LHAASO is an instrument designed for detecting cosmic rays (CRs) and gamma
rays at TeV to PeV energies. The decays of heavy dark matter particles in the
Galactic halo may produce high-energy electrons that can be detected by LHAASO.
The main background for the LHAASO's CR electron measurements is the hadron
residuals due to mis-identification of the particle species. In this paper, we
estimate the LHAASO's electron background using the known all-particle CR
spectrum and the hadron rejection efficiency of LHAASO. With the estimated
background, we predict the capability of LHAASO to constrain DM decay lifetime
at 95% confidence level for various channels. We find that, if neglecting
systematic uncertainties, the CR electron measurement by LHAASO can improve the
current best results by up to on order of magnitude for DM masses between 100 -
1000TeV. However, indirect measurements of CR electrons by ground-based
experiments suffer from uncertainties included in the calculation, the
projected constraints will be largely weakened. So for using the CR electron
observation of LHAASO to constrain the DM parameters, the key point is whether
the systematic error can be effectively reduced.Comment: 11 pages, 8 figures, accepted for publication in PR
Strong enhancement of chlorophyll a concentration by a weak typhoon
Recent studies demonstrate that chlorophyll a (chl a) concentrations in the
surface ocean can be significantly enhanced due to typhoons. The present study
investigated chl a concentrations in the middle of the South China Sea (SCS)
from 1997-2007. Only the Category1 (minimal) Typhoon Hagibis (2007) had a
notable effect on the chl a concentrations. Typhoon Hagibis had a strong
upwelling potential due to its location near the equator, and the forcing time
of the typhoon (>82 h) was much longer than the geostrophic adjustment time
(~63 h). The higher upwelling velocity and the longer forcing time increased
the depth of the mixed-layer, which consequently induced a strong phytoplankton
bloom that accounted for about 30% of the total annual chl a concentration in
the middle of the SCS. The implication is that the forcing time of a typhoon
should be long enough to establish a strong upwelling and consequently for the
induction of significant upper ocean responses.Comment: Typhoon-Ocean Environment interaction
Detection of gamma-ray emission from the Coma cluster with Fermi Large Area Telescope and tentative evidence for an extended spatial structure
Many galaxy clusters have giant halos of non-thermal radio emission,
indicating the presence of relativistic electrons in the clusters. Relativistic
protons may also be accelerated by merger and/or accretion shocks in galaxy
clusters. These cosmic-ray (CR) electrons and/or protons are expected to
produce gamma-rays through inverse-Compton scatterings or inelastic
collisions respectively. Despite of intense efforts in searching for
high-energy gamma-ray emission from galaxy clusters, conclusive evidence is
still missing so far. Here we report the discovery of MeV gamma-ray
emission from the Coma cluster direction with an unbinned likelihood analysis
of the 9 years of {\it Fermi}-LAT Pass 8 data. The gamma-ray emission shows a
spatial morphology roughly coincident with the giant radio halo, with an
apparent excess at the southwest of the cluster. Using the test statistic
analysis, we further find tentative evidence that the gamma-ray emission at the
Coma center is spatially extended. The extended component has an integral
energy flux of in the
energy range of 0.2 - 300 GeV and the spectrum is soft with a photon index of
. Interpreting the gamma-ray emission as arising from CR proton
interaction, we find that the volume-averaged value of the CR to thermal
pressure ratio in the Coma cluster is about . Our results show that
galaxy clusters are likely a new type of GeV gamma-ray sources, and they are
probably also giant reservoirs of CR protons.Comment: 10 pages, 10 figures, Accepted by Physical Review D, more spatial
models for the gamma-ray emission are used, systematic checks on the results
are adde
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